1. Field of the Invention
The present invention relates to a volume holographic memory and an optical information recording/reproducing apparatus incorporating the volume holographic memory.
2. Description of the Related Art
A holographic memory system has been known as a digital recording system to which the principle of holography is applied. The holographic memory system is a system for recording/reproducing digital data with respect to a holographic memory medium (for example, a photorefractive crystal member, such as LiNbO3). The holographic memory system is able to record/reproduce data in units of two-dimensional plane pages. Moreover, the holographic memory system is able to perform a multiple recording operation using a plurality of pages. A structure that the memory medium is formed into a three-dimensional shape, such as a rectangular parallelopiped, to enable a three-dimensional recording operation to be performed is called volume holographic memory (Hereinafter, it is referred to as a xe2x80x9cvolume holographic memoryxe2x80x9d or simply referred to as a xe2x80x9crecording mediumxe2x80x9d.). The general outline of the volume holographic memory system will now be described with reference to FIG. 1.
Referring to FIG. 1, an encoder 25 produces unit-page series data by sorting data to be recorded on a volume holographic memory medium 1, into data corresponding to two-dimensional plane pages, for example, 640 bits wide by 480 bits high data array. The unit-page series data is transmitted to a SLM (Spatial Light Modulator) 12.
The SLM 12 has a modulating process units whose size is 640 bits wide by 480 bits high. Each of the modulating process units corresponds to the unit page. The SLM 12 light-modulates an applied signal beam in accordance with unit-page series data transmitted from the encoder 25. Then, the SLM 12 introduces the modulated beam (called xe2x80x9csignal lightxe2x80x9d) into a lens 13. Specifically, the SLM 12 responds to a logical value xe2x80x9c1xe2x80x9d of the unit-page series data, which is an electric signal to permit passing of the signal beam. On the other hand, the SLM 12 responds to a logical value xe2x80x9c0xe2x80x9d to cut the signal beam off. Thus, electro-optic change of unit-page data according to the contents of each bit can be achieved. As a result, a modulated signal beam serving as signal light for the unit-page series can be produced.
Signal light is made incident on the recording medium 1 through the lens 13. In addition to signal light, reference light is made incident on the recording medium 1 such that angle xcex2 (hereinafter called xe2x80x9cincident angle xcex2xe2x80x9d) is made from a predetermined reference line perpendicular to the optical axis of the signal beam.
Signal light and reference light interfere with each other in the recording medium 1. Produced interference fringes are recorded in the recording medium 1 so that data is recorded. When reference light is made incident at changed incident angles xcex2 to record a plurality of two-dimensional planar data items, three-dimensional recording of data is permitted.
When recorded data is reproduced from the recording medium 1, only reference light is made incident on the recording medium 1 at the same incident angle xcex2 as that employed in the recording operation. That is, signal light is not made incident as distinct from the recording operation. As a result, diffracted light obtained from the interference fringes recorded in the recording medium 1 is allowed to pass through a lens 21, and then introduced into a CCD (Charge Coupled Device) 22. The CCD 22 converts the intensity of incident light into the intensity of an electric signal to output, to a decoder 26, an analog electric signal having a level corresponding to the brightness of incident light. The decoder 26 makes a comparison between the analog signal and a predetermined amplitude (a slice level) to reproduce corresponding data xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d.
The volume holographic memory medium performs a recording operation by using the two-dimensional planar data series as described above. Therefore, when the incident angle xcex2 of reference light is changed, an angle-multiple recording operation can be performed. That is, when the incident angle xcex2 of reference light is changed, a plurality of two-dimensional planes serving as recording units can be provided in the holographic memory medium. As a result, a three-dimensional recording operation can be performed.
In general, the angle-multiple recording operation system uses a pair of galvanomirrors. Angle-multiple recording of a type using the galvanomirrors has been disclosed in Japanese Patent Laid-Open No. 5-142979 and Japanese,Patent Laid-Open No. 10-97174.
The galvanomirrors, however, require a large area of occupation in the system. Therefore, the overall size of the system cannot easily be smaller. What is worse, the system must have a large number of parts used. Thus, the cost cannot easily be reduced.
The limit value of the angle resolution of the galvanomirror is lower than the resolution of the holographic memory medium. Therefore, the system incorporating the galvanomirrors cannot maximally use the recording resolution of the holographic memory medium.
An object of the present invention is to provide a volume holographic memory with which the size of the storage apparatus can be reduced and dense recording can be performed.
To achieve the foregoing object, according to a first aspect of the present invention, there is provided a volume holographic memory including a recording member for recording three-dimensional distribution of interference fringes generated by interference between signal light and reference light, wherein the recording member is in a shape of a cylinder.
Thus, rotation and movement of the volume holographic memory enables multiple recording to be performed without a necessity of adjusting the angle of reference light.
In the above-stated volume holographic memory, if the optical axis of the volume holographic memory coincides with the axial direction of the recording member, rotation of the volume holographic memory around the optical axis enables spatial-multiplexing recording to be performed.
A correcting lens medium may be added to the above-stated volume holographic memory. The correcting lens is made of material having the same refractive index as that of the recording member, and it is in a shape of a rectangular parallelepiped. Furthermore, the correcting lens medium is disposed in such a manner that the recording member is placed in the correcting lens medium.
Therefore, distortion of an image occurring due to a lens effect of the cylindrical recording member can be corrected.
The correcting lens medium may be provided with: a transparent container in which the recording member is placed and which is integrated with the recording member; and a fluid substance which has the same refractive index as that of the recording member, and which is placed in a space between an outer wall of the recording member and an inner wall of the transparent container. Alternatively, the correcting lens medium may be provided with: a pair of lenses disposed to sandwich the recording member in a direction perpendicular to a central axis of the recording member. These correcting lens mediums are able to remove the lens effect of the memory.
Furthermore, a recording member in the shape of a cone may be used as a recording member of a volume holographic memory according to the present invention.
According to a third aspect of the present invention, there is provided an optical information recording/reproducing apparatus comprising: an interference-fringe generating device that generates interference fringes formed by interference between signal light and reference light; a volume holographic memory that records thereon three-dimensional distribution of the interference fringes generated by the interference-fringe generating device; a detecting device that detects diffracted light obtainable from the volume holographic memory by irradiating the volume holographic memory with the reference light; and a reproducing device that reproduces the signal light from diffracted light detected by the detecting device, wherein the volume holographic memory comprises a recording member in a shape of a cylinder.
The optical information recording/reproducing apparatus structured as described above generates interference fringes owing to signal light and reference light. The interference fringes is recorded in the volume holographic memory having the recording member in the shape of a cylinder. When a reproducing operation is performed, the volume holographic memory is irradiated with the reference light to detect diffracted light obtainable from the volume holographic memory. Then, the signal light is reproduced from diffracted light.
A first moving device and a second moving device may be added to the aforementioned optical information recording/reproducing apparatus. The first moving device moves the recording member in an axial direction of the recording member. The second moving device rotates the recording member around an axis of the recording member. As a result, spatial multiplexing recording and angular multiplexing recording can simultaneously be performed with respect to the volume holographic memory medium.
In addition, in the aforementioned optical information recording/reproducing apparatus, a recording member in the shape of a cone may be used as the recording member of the volume holographic memory.